The Winchester Company introduced the 401 WSL in 1910 for the Model 1910 SL rifle,Please note, again: SL = Self-Loading or Semi-Auto.The 401WSL was more powerful than the 351 WSL and suitable for deer hunting;however, the rifle was discontinued in 1936, and the 401 WSL cartridges, was produced until app. early 1950s. Below 3 different Head Stamps